1. Technical Field
The present invention relates to a carbon dioxide separation system for a fuel cell system and, more particularly, to a device for separating a carbon dioxide gas from a fuel stream in a direct fuel cell system, especially in a direct methanol fuel cell (DMFC) system, which is used to supply power to a mobile electronic device.
2. Related Art
FIG. 1 is a schematic diagram of a direct methanol fuel cell (DMFC) system disclosed in U.S. Patent Publication No. 2004/016389A1.
Referring to FIG. 1, a fuel cell stack 10 has an air inlet 11 and an air outlet 13. An air pump or fan 12 supplies reaction air to a stack cathode through the air inlet 11. A heat exchanger 50 is mounted in an outlet stream of a fuel cell cathode. A fan 55 is used to cool the heat exchanger 50, leading to a cooling of the outlet stream and a condensation of water. A two phase flow exits the heat exchanger 50 at an outlet 52. Downstream of the heat exchanger 50, a water separator 60 is mounted in order to separate liquid water from an air stream. The separated water is fed back to the anode cycle of the fuel cell system by a condensation pump 70, while the residual air is vented through an air venting outlet 61 to the ambience.
One indispensable function of the DMFC system is the separation of carbon dioxide from the outlet stream coming out of a stack fuel outlet 16. This outlet stream comprises a mixture of methanol, water and carbon dioxide. For a proper function of the fuel cell, the carbon dioxide has to be separated from the stream prior to the recycling of the fuel stream back into the fuel cell stack 10.
An anode cycle for diluted fuel, comprising a carbon dioxide separator 20 mounted downstream from the stack fuel outlet 16, removes carbon dioxide from the reaction stream and vents it to the ambience through a venting opening 21. In a mixer 22, the reaction stream is mixed with pure fuel from a fuel tank 30. A fuel pump 23 feeds the diluted fuel back to a fuel inlet 15 of the fuel cell stack 10.
European Patent Publication EP 1 383 191 A1 discloses another possible embodiment of a carbon dioxide separation device. Here, the carbon dioxide separation is accomplished in a compartment filled with a fuel mixture having an inlet connected to a stack fuel outlet and an outlet connected to the circulation pump. Carbon dioxide bubbles are separated from the fuel mixture by gravity during the stay time of the fuel mixture within the separation compartment. On top of this separation compartment, a water separator is mounted. There are openings between the carbon dioxide compartment and the water separator leading to the feeding of separated carbon dioxide to the air venting outlet and the separated water into the carbon dioxide separation device (back into the anode cycle), both operations occurring through the action of gravity. The main disadvantage of this embodiment is the strong dependence on the orientation of the device, i.e. the device essentially works only in an upright position. This may also pose problems when the separation device has to be integrated into a flat system set-up as required, for example, for a notebook docking station.
Another embodiment of a carbon dioxide separation device is disclosed in U.S. Pat. No. 6,869,716. Here, the gas separation takes place across a hydrophobic membrane which forms a conduit component around the fuel stream of two phase fluid containing fluid and carbon dioxide. The backpressure needed to press the carbon dioxide through the separation membrane is formed either by a cone-type design of the hydrophobic conduit or by a hydrophilic passageway at the outlet end of the conduit.
The problem with that solution is that it is difficult to manufacture and system-integrate such a tubular hydrophilic membrane with a small diameter. Due to the condensation of water in the compartment which transfers the carbon dioxide to the ambience, the relatively narrow channels might be blocked.
In general, the diffusion rate of a gas across a porous membrane is essentially proportional to the pressure difference between both faces of the membrane, i.e., a higher pressure difference allows for a smaller membrane area, and thus a smaller separation device.
The presented means for creating backpressure only lead to a limited pressure, and thus to a requirement for a relatively large area of separation membrane.